The main body of the St Mary's Church was built in the late 12th century and was probably rebuilt in the 1240s, including a tower, porches and north aisle, after being damaged by Llewelyn the Great in 1220. An upper level of clerestory windows was added during the 15th century. The tower originally had a timber spire clad in lead, though it was removed in 1802 after becoming dangerous.
The church's interior has a fine timber panelled ceiling with moulded beams, rafters and ribs, and carves bosses at the intersections. It dates to c. 1500.
The church organ dates from 1737, by Harris & Byfield, with two keyboards.
References:The Clementinum is a historic complex of buildings in Prague. Until recently the complex hosted the National, University and Technical libraries, the City Library also being located nearby on Mariánské Náměstí. The Technical library and the Municipal library have moved to the Prague National Technical Library at Technická 6 since 2009. It is currently in use as the National Library of the Czech Republic.
Its history dates from the existence of a chapel dedicated to Saint Clement in the 11th century. A Dominican monastery was founded in the medieval period, which was transformed in 1556 to a Jesuit college. In 1622 the Jesuits transferred the library of Charles University to the Klementinum, and the college was merged with the University in 1654. The Jesuits remained until 1773, when the Klementinum was established as an observatory, library, and university by the Empress Maria Theresa of Austria.